The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with in-vivo examinations and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in conjunction with spectroscopy examination of localized regions for imaging, chemical shift analysis, quality assurance, drug flow tracing, and the like.
When examining a complex structure, such as a region of a human patient, resonance may be excited in numerous different chemical compounds. To isolate a small volume of the region of interest or voxel, two techniques are commonly employed--phase encoding techniques and voxel localization techniques. In phase encoding techniques, phase encoding gradients are applied across the sample such that the resonance signals emanating from various locations across the sample are spatially encoded by this phase. In voxel localization, the signal is only recovered from a selected small volumetric element or voxel.
More specifically, phase encoding techniques typically provide spatial definition over a region whose size is defined by the sensitive volume of the receiver coil. Because the sensitive volume is determined by hardware geometry, it is substantially fixed from study to study with little attitude for adjustment.
One of the problems with phase encoding techniques is that they are susceptible to errors resulting from layers of different tissue types. The boundary layers between materials with different magnetic susceptibilities causes large magnetic field inhomogeneities that degrade water suppression and spatial resolution. Further, it is desirable to avoid the excitation of resonance in certain regions. Specifically, fat layers that surround the skull and other regions of the body commonly produce much larger magnetic resonance signals than metabolites of interest. The fat signal response can obscure the spectra of voxels of interest due to leakage of the point-spread function from neighboring voxels. In this manner, the fat signals degrade the dynamic range of the resultant signals limiting the ability to detect low concentrations of metabolites.
Voxel localization techniques, such as the technique described in U.S. Pat. No. 4,771,242, recover the signal from only a small voxel in the sample. To map or measure chemical concentrations over an extended region, a plurality of single voxel experiments are conducted. In each repetition, the voxel is defined at different location within the sample. Although this technique is accurate, it tends to be time consuming.
In another voxel localization technique described in "In-vivo 1H NMR Spectroscopy of the Human Brain by Spatial Localization and Imaging Techniques", by P. Luyten, et al., SMRM Book of Abstracts, page 327 (1988), a volume is selected with refocused stimulated echoes. This volume selection technique utilizes a series of 90.degree. pulses. Phase encoding for one or two dimensional spectroscopic imaging is combined with the volume selection in order to suppress unwanted lipid signals from surrounding tissue. Although subvoxels can be defined within a relatively large voxel, this technique has several drawbacks. First, the 90.degree. stimulated echo sequence recovers only half the signal. The other half is lost.
Secondly, these voxel techniques require three RF pulses in order to localize the voxel. Because the first RF pulse initially excites resonance, the time required for the following two pulses extends the duration between the first pulse excitation and data collection. Extending this duration limits the recovered signal to the contribution from dipoles with a longer T2 relaxation time.
The present invention provides a new and improved spatially encoded spectroscopy technique which overcomes the above referenced problems and others.